1. Field of the Invention
This invention relates to an optical wavelength converting method for converting a fundamental wave into a second harmonic or the like by the use of a Cerenkov-radiation type optical wavelength converter device, and more particularly to such a method in which a blue region wavelength-converted wave can be obtained at higher efficiency.
2. Description of the Prior Art
Many of conventional silver halide photosensitive materials which are used for a color printer or the like exhibit spectral sensitivity properties shown by curve a in FIG. 2 in the blue region. As can be understood from the curve a, many of silver halide photosensitive materials are remarkably sensitive to light having a wavelength near 480 nm and their spectral sensitivity peaks at about 470 nm. Recently, silver halide photosensitive materials which exhibit high sensitivity to light having a wavelength up to about 500 nm as shown by curve b in FIG. 2 and conform to a 488 nm blue laser beam emitted from an Ar-laser have been available.
When light having a wavelength shorter than 430 nm is used as a recording beam, there arise problems with respect to absorption in gelatin, limitation on spectral sensitizer, stability of color images and the like. Accordingly, when a color image is recorded on a silver halide photosensitive recording medium by scanning the recording medium with a blue laser beam, it is necessary to use a blue laser beam having a wavelength of 430 to 500 nm in order to obtain a clear and stable color image.
When recording information on an optical disk with a laser beam, the shorter the wavelength of the laser beam is, the smaller the beam spot can be and the denser recording becomes feasible. However, also in this case, a laser beam having a wavelength not shorter than 430 nm must be used at present due to limitation on coloring materials, absorption in the substrate of the disk, and the like.
Further, in the color printer system and the optical disk system, it is preferred that a laser beam having a wavelength of not shorter than 430 nm be used also in view of the transmittivity of the lens.
As described above, a laser beam having a wavelength of 430 to 500 nm provides substantial improvement in performances of various optical systems. However, a blue laser which is compact in size and can be directly modulated has not been put to practical use. Further it is said that a semiconductor laser which emits blue region laser beam will not be put to practical use for the time being.
Under these conditions, various attempts have been made to shorten the wavelength of a laser beam by the use of an optical wavelength converter device. For example, there has been disclosed in U.S. Pat. No. 4,909,595 a Cerenkov-radiation type optical wavelength converter device which is in the form of an optical fiber comprising a core of a single-crystal organic nonlinear optical material and a cladding which surrounds the core. As the organic nonlinear optical material, those of an orthorhombic system of mm2 point group can be used.
By means of a combination of such an optical wavelength converter device and a laser source, a blue laser beam can be obtained relatively easily. However, since the wavelength conversion efficiency of such a system is low, it has been difficult to obtain a blue laser beam having an intensity sufficient for the aforesaid applications.